Help - what can I do to clean this toilet?

[namechanger70]

Hi I am looking for some help. I have been away for a while and returned to one toilet that is brown and discoloured in the bowl and u-bend. It has been cleaned but it looks dirty. I have tried every product I can think of, the coke overnight trick and bleach for days with brillo pad scrubbing and nothing is lifting it although it looks like a good scrub would do it. Can anyone please suggest a magic potion to try? Thanks.

Thanks Lady, when I googled the Kilrock Big K I thought the bottle looked familiar, and I have some in my cleaning cupboard.
Will use it later and report back!

Re the Coke, when I was little my mum poured some on some grimy old coins and said it was the acid in it that burnt the dirt off, and that I shouldn't drink coke because that would happen to my stomach too!

After reading this I did my loo again as the brown scale was starting to creep round the bottom under the water again.
I beg your pardon - it doesn't fizz pleasingly in the loo - I made the water hot in the pan, chucked some down and let it sit for 4 hours. Good scrub round with a brush and it cane away in chunks. Sorted.
(It fizzes like a fantastic science experiment and works in seconds in the kettle - and my kettle is a glass jobby so watching the scale disappear is wonderful)

How did you get on?